1. Field of the Invention
The present invention relates to a method of manufacturing a semiconductor device, in particular, a method for forming an impurity region consisting of a high concentration part and an outer low concentration part for a semiconductor device.
2. Description of the Prior Art
In the prior art, semiconductor devices have been known, for example, MOS transistors of LDD-structure (Lightly Doped Drain-structure), which are characterized by impurity regions consisting of a high concentration part and an outer low concentration part and having a lateral concentration profile.
The impurity region having a lateral concentration profile has the main effect of raising the breakdown voltage of a PN junction. Therefore, the employment of such a lateral concentration profile is effective for the impurity region (called guard ring) formed for the purpose of raising the breakdown voltage of a planar junction. For example, a guard ring employed for an avalanche photo diode (APD) shall be described as follows.
FIG. 1 shows a section of the structure of an APD formed with a guard ring. Reference numeral 1 indicates an N.sup.+ type InP substrate, on which the following are laminated in order: buffer layer 2 of N.sup.- type InP, optical absorption layer 3 of N type InGaAs, buffer layer 4 of N type InGaAsP, multiplication layer 5 of N type InP and N.sup.- type InP layer 6, which is formed with P.sup.+ type main impurity region 13 to form a PN junction of the diode between the P.sup.+ type region and the N.sup.- type InP layer 6. Also, P type guard ring 12 is formed around and in contact with P.sup.+ type main impurity region 13. Further, the PN junction of the main impurity region is formed as a planar type, since a mesa type having a large exposed part in the junction results in deterioration of the junction, leading to lower reliability.
The APD as dsecribed above operates as follows. When light enters the photo detecting part of the PN junction under the condition of applying a high reverse bias near the breakdown voltage, light absorption by optical absorption layer 3 causes the production of carriers (electron-hole pairs). Optical absorption layer 3 is arranged to have a small band gap so as to absorb light of long wave length. One component (hole) of the resultant carriers in optical absorption layer 3 is accelerated toward the PN junction by the electric field of high intensity applied thereto and passes through carrier multiplication layer 4 thereby receiving avalanche-multiplication. The carriers multiplied as such are passed through the PN junction which is broken-down, and are drawn out of electrodes 15 and 16.
Guard ring 12 is formed for the following reasons. A planar type PN junction, although possessing various advantages, poses a problem called "edge breakdown". That is, the junction interface of a corner of the impurity region in its construction forms a curvature with a small radius of curvature to concentrate the electric field in the curvature, such that the curvature breaks down prior to the breakdown of other parts. The APD in FIG. 1 is so arranged that P type guard ring 12 which has higher breakdown voltage covers the corner of the P.sup.+ type main impurity region to prevent edge breakdown.
For the full function of preventing said edge breakdown, guard ring 12 should preferably have a deep and smooth concentration profile of impurity, in contrast to the abrupt junction of main impurity region 13, which further enhances the difference of the breakdown voltages between guard ring 12 and main impurity region 13. To form a guard ring having such a preferred profile, for example, as in the case of an APD employing a compound semiconductor of a III-V group represented by an InGaAs/InP system, Be ion implantation is used to form P type guard ring 12, whereas Cd is used as an impurity for forming P.sup.+ type main impurity region 13. The ion implanted Be has a deep diffusion depth that enables the guard ring to obtain the preferred profile.
However, to further enhance the function, guard ring 12 is required to have a concentration profile not only for the depth direction but also for the lateral direction. For example, as shown in FIG. 1, guard ring 12 is designed to comprise P type region 12a and outer P.sup.- type region 12b to further enhance the breakdown voltage of the guard ring. Yet, even with the Be ion implantation that obtains the preferred depth-oriented profile, the lateral diffusion length of Be is not so great that a preferred smooth concentration profile can not be obtained.
Therefore, to obtain a guard ring having a preferred lateral concentration profile, the following methods have been proposed in the prior art.
The first method was to employ different ion implantation conditions respectively for P type region 12a and the P.sup.- type region 12b of the guard ring
(Japanese Patent Application Disclosure No. 60-198786 and No. 61-101084). According to this method, however, ion implantation must be performed twice to form guard ring 12, with the disadvantage of complicating the process of manufacture. In addition, ion implantations can often cause imperfections in the crystal lattice of the substrate such that a great number of ion implantations will cause a decrease of the breakdown voltage or an increase of the leak current.
The second method proposed was to make etching deeper than the peak depth of the impurity on the surface around the ion implanted region to form a low concentration part of the guard ring (refer to Japanese Patent Application Disclosure No. 60-198785). This method also has a disadvantage of complicating the process that much more, due to the extensive etching and further forming steps on the guard ring surface.
The third method proposed was to have an insulation film of a predetermined thickness distribution for a mask in the course of ion implantation for forming guard ring 12 (Japanese Patent Appl. Disclosure. No. 61-101085). In this method, the control for accurate thickness distribution of the insulation film for mask becomes difficult and also the process becomes complicated, with disadvantage of reducing the yield rate or resulting in high manufacturing costs.